Square wave pulse generating system



June 30, 1953 G. c. SUMMERS SQUARE WAVE PULSE GENERATING SYSTEM 4Sheets-Sheet 1 Filed Jan. 24. 1949 GERALD C. SUMMERS INVENTOR.

ATTORNEY June 30, 1953 G. c. SUMMERS 2,644,130

SQUARE WAVE PULSE GENERATING SYSTEM Filed Jan. 24; 1949 4 Sheets-Shee 2REVERSING 5W|TCH GERALD c. SUMMERS INVENTOR.

ATTORNE Y June 30, 1953 G. c. SUMMERS SQUARE WAVE PULSE GENERATINGSYSTEM Fi l ed Jan. 24, 1949 4 Sheets-Sheet 3 GERALD G. 8U MME R3 1N VENTOR. W 4%.

ATTORAEY G. C. SUMMERS SQUARE WAVE PULSE GENERATING SYSTEM June 30, 19534 Sheets-Sheet 4 Filed Jan. 24, 1949 GERALD C. SUMMERS IN V EN TOR.

ATTORNEY Patented June 30, 1953 UNITED STATES PATENT OFFICE SQUARE WAVEPULSE GENERATING SYSTEM Gerald (J. Summers, Dallas, Tex., assignor, bymesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York,N. Y., a corporation of New York Application January 24, 1949, SerialNo. 72,363

19 Claims. 1

This invention relates to electrical prospecting, more particularly to asystem for generating current pulses of a desired form and frequencywhich are applied to an earth load for the purpose of obtaininginformation as to the character of subsurface strata.

In electrical prospecting methods it has been determined that the pulsesmust be of relatively high power, at frequencies variable in the rangeof 2, 4, 8, 16 cycles per second, more or less. It has heretofore beenproposed to generate such signals by the opening and closing ofcircuitbreaker contacts driven by a variable speed motor. However, whenthe contacts must perform the work of making and breaking the highcurrent flow, a certain amount of pitting and erosion takes place andthe device does not remain precise over an appreciable period. Moreimportantly, any are forming upon the opening of the contacts representscurrent flow after the separation of the contacts. Hence, the length ofeach pulse represents an uncontrolled variable.

It has also been proposed to utilize gas discharge tubes of thethyratron type. However, the wave shape of the pulses from such tubes isnot rectangular and with alternating current successive pulses are notalike except where the ratio of the frequency of the alternating currentsource to the frequency of commutation is a whole number.

In still other systems, an electric commutator comprising gas dischargetubes capable of delivering rectangular pulses has been proposed. insuch systems the termination of a positive pulse coincides with theinitiation of a negative pulse. Such systems do not provide for thespacing of the successive pulses:

In carrying out the present invention in one form thereof, there isprovided a method and system for generating pulses of relatively highpower which are rectangular in shape, which are spaced one from theother, and all of which are of the same shape. More particularly, thereis provided, in circuit with electrodes for applying current pulses tothe earth load, a circult-reversing device which is operated during theintervals between successive pulses. Accordingly, its operation does notaffect the character or timing of the pulses carried thereby. Thecurrent flow is derived from a source capable of producing rectangularpulses under the control of an electric valve having a grid for con-jtrolling the conductivity thereof. The negligible current demand of thegridcircuit makesfeas'ible the use of a mechanical commutator in thatcircuit for controlling the impulses though, in the preferred form ofthe invention, the initiation of each impulse may be mechanical whilethe length thereof is electrically determined.

In a further preferred form of the invention, the length of each pulseis determined by an artificial transmission line having a plurality ofcapacitors arranged to be charged through circuits individual to one ormore capacitors and to be discharged through a separate circuit forminga part of the artificial transmission line. That line may be connectedto the earth electrodes either through a mechanical circuitreversingdevice or through an electronic device, both being contemplated by thepresent invention.

In each form of the present invention the power pulses, becauseidentical and spaced one from the other, make possible the detection ofelectrical signals with such accuracy that important subsurfaceinformation as to the character of subsurface strata may be obtained.The signal-level of such reflected signals is quite low and the changein character, due to subsurface strata of differing electricalproperties, is of course of a still lower order. Hence, it is importantthat the power pulses both as to shape and duration shall be more nearlythe same than the variations in acoustic pulses by which seismicinformation is derived.

For a more complete description of the in vention and for furtherobjects and advanta es thereof, reference should be had to the followingdescription taken in conjunction with the accompanying drawings inwhich:

Fig. l diagrammatically illustrates in simplified form a systemembodying the present invention;

Fig. 2 is a schematic diagram of another form of the invention andincludes a grid-controlled high-vacuum switch as the source of pulses;

Fig. 3 is a schematic diagram of a generating system utilizing a delayline as the source of impulses; and

Fig. 4 illustrates a modification of Fig. 3.

Referring now to Fig. l, the system of the present invention includes asource ii.- of spaced unidirectional low-frequency pulses H. The pulsesare applied to the earth E by way of electrodes l2 and I3. Acircuit-reversing device such as a reversing relay or switch M; isconnected between the electrodes I2 and i3 and the source ofunidirectional pulses for reversing the connections between the sourcel0 and the electrodes to convert the unidirectional pulses l l to pulsesi ib alternately of opposite polarity. The unidirectional pulses H, eachof rectangular shape, and each spaced an interval one from the other,are obtained by reason of flow of current from a unidirectional source,such as a battery i5 having adequate capacity to supply unidirectionalpulses. The battery is connected to the anode I6a of an electric valveis having a cathode it?) and a control grid Ito. The grid circuitincludes a grid resistor IQ, a bias battery 20, a resistor 2| and asecond bias battery 22, all connected in series between the grid and thecathode Hill. A pair of conductors 23 and 24 form a part of the circuitof a control switch 25 for removing from the circuit the battery 22which normally negatively biases the valve it to prevent flow of currenttherethrough. The switch 25 is operated by a cam is driven by a variablespeed driving device such as a motor 2! drive-connected to cam asthrough a speed-changing device 28 such as Reeves drive having a controlcrank or wheel 29. The motor 27 also drives through device 25 a cam 30in timed relation with the cam 25 in order to operate the reversingrelay l4 during intervals between each of the rectangular pulses l l.

Assuming the line switch 3! of motor 2i closed and the motor rotating ina direction to drive the cams in a counter-clockwise direction, it willbe seen that the upper contacts of switch 25 are closed. With the uppercontacts of switch 225 closed, the bias battery 22 and the resistor 2!are effectively removed from the circuit. Accordingly, the bias battery28 effectively applies a positive potential in the grid circuit torender conductive the valve i6. Current flows from battery 15 throughvalve Hi, by way of conductor 55a to the lower contact of switch Hi, andthence to the earth electrode I3. is by way of the other earth electrodeiii, the upper contact of reversing switch Hi, and thence by conductor52) to the battery 15.

The current rises immediately to its maximum value and flows for a timeinterval determined by the length of time the switch 25 remains in itsuppermost circuit-closing position. For the pur pose of the presentdescription and not by way of limitation, cam 26 shaped as illustratedmaintains the switch 25 closed through 70 rotation. Thereupon, theoperation of the cam 26 opens the by-pass circuit through its uppercontacts to make effective in the control circuit of valve it thepowerful bias battery 22 to render non-conductive valve it immediatelyto interrupt the current flow. In this manner the first electricalpulse, of seventy degrees duration, is applied between electrodes l2 andI3. There is then provided a time interval by travel of the cam 26through one hundred and ten degrees, at which time the outer level ofcam 2t operates switch 25 to its lowermost position.

The reversing switch i4 is operated under the control of thecam-operated switch 33 arranged to control energization of its operatingcoil 33 from supply lines 35 and 36. More specifically, after switch 25has opened to render valve l5 non-conductive the cam 39, during rotationof cam 2:} through its one hundred and ten degree section, closes switch33 to energize the operating coil 24;. The reversing switch it is thenoperated from one circuit-closing position to a reverse circuit-closingposition during the interval of no current flow following the first ofpulses I l. Accordingly. there is no chance for arcing at the The returncircuit contacts of switch l4. There is neither erosion nor pitting ofthe contacts. Even if there were, it would have no effect upon the waveshape or time of the current of the next cycle since initiation andtermination is under the control of the electric valve [5.

With the reversing switch M in its upper position to reverse thedirection of current flow through earth electrodes l2 and E3, the cam 25operates the switch 25 to its lowermost circuitclosing position, againto remove the bias battery 22 and resistor 2i from the grid circuit ofthe valve l6 and to apply the positive bias of battery 20 thereto.Current again flows from battery I5 to produce the second of therectangular pulses II. The outer level of cam 25 also extends forseventy degrees so that the second pulse is identical in length with thefirst. The cam 25 then moves switch 25 to its intermediate open-circuitposition to make battery 32 ei'lective to render the valve I6non-conductive. In the subsequent interval of no current flow ascontrolled by the additional one hundred and ten degree inter-- mediatelevel of cam 26, the so is rotated to open the switch 33 to deenergizerever ing coil 34. The switch [4 thereupon reverses the current path tothe current electrodes and It during a period of no current flow.

The foregoing operations are then repeated to produce a series ofidentical current pulses ll, each of rectangular shape and each spacedone from the other by a predetermined time interval. During each suchtime interval the reversing switch I is moved first to one and then tothe other of its positions to convert the identical rectangular pulsesof unidirectional polarity to pulses Mb successively of oppositepolarity. The succession of pulses of alternating polarity are appliedto the earth through the current electrodes l2 and [3.

By means of detecting electrodes 3! and 33, located a substantialdistance from the current electrodes, electrical signals 33 are detectedand recorded on a device 48, the recorded signals being useful inyielding information as to the character of the subsurface strata. Aspointed out in Clewell Patent No. 2,454,911, the reflected signals suchas appear in the interval between current pulses 141), or variations inthe electrical field, detected or received by electrodes 3? and 38 areexceedingly small in magnitude, of the order of millivolts. Accordingly,the device id preferably includes suitable amplifying equipment toelevate the detected signals to a magnitude adequate for actuation ofthe recording device. Since the signals themselves are small and requiregreat amplification, it will be apparent that the pulses I42) appliedto, the earth must have great uniformity. If successive pulses dinerfrom preceding pulses, the differences themselves might account forvariations in the electrical field which could be confused with signalsdue to subsurface anomalies. Thus, with pulses of varying character thesystem as a whole would not be satisfactory. The impcrtance of theapplication to the earth of uniform signals produced in accordance withthe present invention not be overemphasized.

Though as above described each of the pulses l I has a length of seventydegrees, it is to be understood they may be of any selected length orduration. The cam 26 may be adjustable to change the relative lengths ofthe three levels thereof or different cams may be substituted therefor.Similarly, cam 30 may be adjustable or it may be replaced or angularlyadjusted on the drive shaft to produce operation of reversin switch [4during a period of no current flow. The switches and 33 are preferablybiased against cams 26 and though they may be mechanically connectedthereto for positive actuation in both directions if desired.

By adjusting the crank or wheel 29, the speed of the drive shaft may bevaried or where a variable speed motor is used, the device 28 may bedispensed with. The pulses ll may be produced at any selected rate fromaround one-half cycle per second to thirty cycles per second, each cyclerepresenting a positive pulse, an interval, a negative pulse and asecond interval. plained in said Clewell patent, the electrical surveymay be conducted by varying the frequency through a selected range andnoting the character of the detected signals. For some locations signalsof one length may be preferred over signals of a different length.Nevertheless, these severe requirements are satisfactorily met by thepresent invention.

As further explained in said Clewell patent, the signals 39 are detectedduring selected time intervals preferably in an intermediate portion ofeach interval between pulses Mb- This may be readily accomplished by acommutator driven in synchronism from or by the drive shaft as indicatedby the dotted line 4!. Referring now to Fig. 2, a modified form of theinvention has been shown for producing pulses Mb between the earthelectrodes 12 and I3. A plurality of electric valves or triodes 16 areconnected in parallel and are so controlled as to function as a precisecircuit controller. Each electric valve or triode I6 is provided with ananode !6a, a cathode [6b, and a control grid 160. The anodes areconnected in parallel with each other and in series with a source ofunidirectional current such as the battery IS, the circuit beingcompleted through a reversing switch I4, electrodes l2 and 13, and thecathodes I! which are also connected in parallel with each other.

The grids 160 of the triodes l6 are also connected together and to acommon grid-controlling circuit which, it will be observed, includes agrid resistor 19, a source of biasing potential such as the battery 20,a grid resistor 2|, and a second source of biasing potential such as thebattery 22. As in Fig. 1, the resistor 2| and the battery 22 may beeffectively inserted and removed from the grid-control circuit byclosure of a control switch 42 to complete a by-pass circuit. Thebattery is not short-eircuited by switch 42 due to the relatively highvalue of the resistor 2!.

Though vacuum tubes or electric valves of other types may be utilizedproviding they have suitable characteristics, triodes of the GAS'l-Gtype have been found particularly suitable for the invention. By using asufiicient number of such triodes in parallel, the voltage drop betweenthe anodes and cathodes, as for example between the output conductors 43and 44, when the current is of the order of two amperes, will besatisfactorily low; that is, about fifty-five volts for twelve triodes.By utilizing the high vacuum type of tube, there is attained not onlycontrol of the initiation of each pulse, but also control or thetermination thereof. The low plate drop of the tube also makes itparticularly desirable in conjunction with a direct-current source forthe production of spaced rectangular pulses without prohibitive powerloss in the tube. This will be recognized as important since electricalAs exprospecting for oil generally takes place over areas remote fromsources of current and it is,

therefore, necessary to relyupon transporta-v ble sources such asstorage batteries and the like. However, such operational limitations onthe equipment are satisfactorily met by the present invention.

The parameters of the grid-control circuit 45, for satisfactoryoperation, may not be deter mined by the usually accepted empiricalrelations. They would indicate that for an anode voltage, battery 20 of230 volts, the cut-off bias would be of the order of 142 volts. For the6AS7-G type tube whose amplification factor is 2.1, it has been found,where the plate voltage is 230 volts, that inorder to meet therequirements of the present invention it is necessary to utilize aneffective negative bias from battery 22 of at least 200 volts. Suchnegative bias is necessary to reduce the plate current to a value belowthat which would prevent interference with, or effect upon, detectedsignals. A reduction below one microampere has been found satisfactory.Moreover, it has been found that a valve or triode I6 is not effectivefor production of rectangular current pulses of uniform current valueunless the grid during each period of conduction is made positive.Accordingly, the battery 20 is necessary to yield rectangular pulseshaving fiat tops indicating lack of current variation during eachconductivity period. A battery 23 of 22 volts has been utilized andfound to be satisfactory in conjunction with a battery 22 or othersource of biasing potential of the order of 2'10 volts. The'resistor IQof one-tenth ofa megohm and the resistor 2| of one megohm weresatisfactory. The resistance values of resistors l9 and 2| are notparticularly critical. Those given are exemplary and satisfactory. Theswitch 42 is preferably of the type in which the contact carrying arm42a. is biased as by spring 42b in its closed psition. Additionallythere is included a roller member, preferably a small ball bearing 42c,carried by contact arm 42a. Cam '46 operatively engaging bearing 42cserves to open switch 42a. By adjustment of the distance between shaft46a carrying cam 46, and arm 42a, the period of closure of switch duringeach revolution of cam 46 may be selected and/or controlled.

With the foregoing arrangement, the current flowing through the controlswitch 42, when closed, was less than 500 microamperes, a whollynegligible value as regards wear or arcing, or other damage to theswitch contacts. Accordingly, the switch 42 may be and has been operatedby the cam 45 over long periods of time with great precision, and withnegligible wear and no arcing or pitting of the contacts. 7

The cam 46 is driven by the motor 21 under the control of thespeed-changing device 23. The motor and speed changer also drive the cam47 which controls the operation of the switch 33 for energization of theoperating coil 34 of the reversing switch hi. The cam 46 is spaced fromthe contact arm 42a periodically to close and to open the switch 42 forproduction of the rectangular pulses I l with selected time intervalstherebetween. Similarly, the cam 47 is spaced from its associatedcontact arm for closure and opening of the switch 33 during a timeinterval between the rectangular pulses I l. The drive of the cams 46and 47 in timed relation one with the other results in the series ofpulses 1%, each of identical shape with the other and of alternatingpolarity. I

From the foregoing it will be appreciated that even though the systemsof Figs. 1 and 2 may be satisfactory, the leave something to be desiredbecause of the voltage and power limitation of the vacuum tubes such asvalves It. In the further modification of the invention illustrated inFig. 3, a system is disclosed in which the pulses of greater powermagnitude are produced, the duration of each pulse being independent ofany switch and dependent entirely upon circuit constants of apulse-producin network. The foregoing is accomplished by the provisionof a trans mission line 50 formed by a plurality of condensers 5|, eachof which has one terminal connected to a common output conductor 52. Theother terminals of the condensers 5| are interconnected throughseries-inductances 53 to the other output conductor 54. The line isconnected by conductor 54 and a gas discharge tube or thyratron tooutput terminals 55 and 5'! leading to the reversing switch M. Theelements forming the line 50 are chosen in number and size to form atransmission line having a time constant equal to one-half the period ofthe desired pulse length. If such a line is terminated in an impedanceequal to the surge impedance of the line, a current pulse will flowthrough the load which will be rectangular in shape upon discharge ofthe line. If the line condensers 5| are charged and the thyratron 55 isinitially non-- conductive, a trigger voltage applied to the gridrenders it conductive and initiates the discharge. Following theinitiation of discharge, a transient travels down the line toward theopen end. The transient is reflected at the open end out of phase andupon arrival at the output end causes the plate voltage of the thyratron55 to be Zero. Consequently, the thyratron becomes non-conductive andthe flow of current to the earth load connected to the terminals 55 and5'! is abruptly terminated.

In order to provide sufficient capacity in the line to deliver highcurrent pulses and to provide for the charging of the line during eachpulse interval, which in some instances may be of the order of thedischarge period, special considerations must be given to the chargingcircuit. In accordance with the present invention, a charging circuit isprovided for each of the condensers or capacitors 5| of the transmissionline 50. The charging circuit includes a source of D. C. potential 58connected through a switch 59, operable by cam 60, to th plates of aplurality of diodes 6|. The plates of the diodes are connected to getherand to the switch 55 b a conductor 52. The cathode of each of the diodes6| is connected to one terminal of each of the condensers 5!. Uponclosure of the switch 59, as by the cam Bil, current flows to thecondensers 5 I, charging them to a potential approaching that of thesource 58.

Inasmuch as all condensers 5| are simultaneously charged throughparallel circuits, each including a diode 6|, and each directlyincluding a capacitor to the exclusion of the associated inductances, itwill be seen that all capacitors or condensers 5| at any instant duringthe charging period will be at the same potential. If the line 50 wereto be charged from one or both ends, there would be present during thecharging period different voltages at the several condensers 5|. Theline 50 would not be stable. However, by using the separate chargingcircuits, the line 50 is at all times stable. The charging circuit maybe opened at any time with assurance the condensers will be uniformly orequally charged with a resultant stabilized transmission line 5|! readyfor discharge to produce the next earthexploring impulse. Hence, thecharging period may be less than the length of an impulse.

The use of a transmission line also makes possible a high voltage sourceof supply 58. A rectiher or other source may have a voltage of the orderof three thousand volts or more. The higher voltage of the sourceproduces impulses of greater magnitude. The greater the electricalenergy delivered to the earth, the greater will be the magnitude of thereflected or detected signals. Improved records and depth-ranges arethereby secured. Since the source 58 is not connected to the line 50during discharge, its regulaion does not affect the character of thepulse.

The output of the line 50 is connected by way of a reversing switch I4to earth electrodes. As illustrated, the points 56 and 51 are connectedto one pair of terminals of the reversing switch M. The switch l4reverses circuit connections for flow of current in the earth circuitduring intervals between the pulses delivered by the line 50. Moreparticularly, the switch M is reversed by energization of its operatingcoil 34 upon closure of the switch 33. The switch 33 is actuated by thecam 47 driven by a variable speed motor 21a. The motor 21a also drivesthe cams 6i) and 63, the latter operating a switch 64 to control thegrid circuit of the thyratron 55 at predetermined instants. The gridcircuit of the thyratron 55 includes'a resistor 65 and source ofpotential such as a bias battery 66 connected in series between the gridand cathode. The switch 55 connected between the grid and cathode of thethyratron 55 effectively removes the bias battery 56 from the gridcircuit to initiate discharge of the line 5!).

The operation of the thyratron 55 in conjunction with the line 50 issatisfactory while generally they are not satisfactory for control ofinitiation and termination of impulses. More particularly, when a highvoltage source of the order of a thousand volts, is used, there is noproblem as at low voltages, or delay, in ionizing the thyratron even atrelatively low temperatures. Hence, in conjunction with a high voltagetransmission line the thyratron has proved to be a satisfactoryelectronic switching device.

Assuming now that condensers 5| are initially charged, the switches asillustrated in Fig. 3, and the motor 27a operating at a selected speed,the cam 60 will open switch 59 to interrupt or disconnect the chargingcircuit. The cam 63 operates the switch 64 to remove the negative biason the grid of the thyratron 55. The thyratron is thereby madeconductive for flow of current from output conductor 54 of thecapacitive storage elements 5| in the line 50 through thyratron 55 tothe electrodes l2 and I3 and thence to the other output conductor 51.The current flows for a period equal to twice the electrical length ofthe transmission line 50. Following termination of the pulse, thecharging circuit is again closed through switch 59 to recharge each ofthe condensers 5| to the voltage of the source 58 or to a predeterminedfraction thereof. Simultaneously, and during the charging period, switch64 is momentarily closed to cause the switch M to reverse theconnections between the output conductors 54 and 5'1 and the electrodesl2 and I3. Thereafter, the switch 59 is opened and switch 64 closed todischarge the line a second time, the direction of current flow betweenthe electrodes l2 and I3 being reversed from the '9 direction of thefirst pulse. Following the reversed polarity pulse, the condensers ofline 50 are again recharged, the switch I4 again actuated, and theswitch 64 closed for delivery of a pulse of opposite polarity to theearth load. The operation continues, as described.

In the foregoing operation of the system, it is to be understood thevariable resistor 67v has been set to a value such that thecharacteristic impedance of the line 50 is equal or substantially equalto that of the earth load between electrodes I2 and I3. or theresistance of the earth between points spaced, for example, 500 apart isin the order of 1 ohm. However, using electrodes of practical nature(porous pots containing a copper sulphate solution) tests made overwidely scattered areas indicate that the resistance between suchelectrodes I2 and I3 spaced 500 feet apart is generally slightly lessthan 100 ohms. Accordingly, the line 50 is designed for a characteristicimpedance of around 100 ohms. The resistor 61 is then adjusted to add tothe earth load resistance increasing the impedance of output circuitsubstantially to equal the line impedance of 100 ohms. In this mannerthe line 50 may be used over widely distributed areas without changingthe circuit constants and yet with matched impedance for best operatingconditions.

Where the impedance match between line and load is close, the line willcompletely discharge to produce a rectangular pulse. Hence, the pulsewill be self-terminating. However, if there is slight mismatch, therewill be continued current flow after expiration of the selected pulseperiod,

the thyratron 55. A reverse mismatch tends to I produce current flow inthe opposite direction. However, in both cases the magnitude of currentflow through the thyratron drops very rapidly. As soon as the voltagedrops below the thyratron ionizing voltage (approximately twelve volts),the thyratron 55 becomes non-conductive. Since the deionization time ofthyratrons is in the order of .001 second it is preferable that the loadimpedance is adjusted to equal or be slightly less than the surgeimpedance of the line to assure an abrupt termination of the pulsecurrent and following quiescent period. This permits detection ofreflected signals in. the pulse interval unaffectedby any continuedcurrent flow between electrodes I2 and I3. The detected signals forpulses of two amperes at two. hundred volts vary from around five totwenty or more micro volts depending upon the electrical characteristicsof the subsurface strata. For pulses of higher power, as from thetransmission line, the received signals will be of correspondinglygreater magnitude. The condensers used in line 50' may permit a voltageof the order of several thousand volts which results in a decidedincrease in pulse power.

It has been found that for a given frequency, the relative values of thehigh-frequency cornponents and low-frequency components of the energydelivered to the earth are determined by the pulse length. For longerpulse lengths, the lowfrequency energy is predominant and permitsgreater penetration in the earth. The transmission line included in thepresent invention may be operated, as by suitable selection. of circuitparameters, to produce pulses of variable length.

Theoretically, the earth loadv As those skilled in the art understand,the pulse length for any given line will be determined by the timeconstant of its constituent impedances. Factors to be considered indesigning a line of the desired pulse length include the impedance intowhich the line is to work and the magnitude of the current desired. In atypical embodiment of the invention for producing electrical impulseseach of rectangular shape and of 10 milliseconds duration, thecharacteristic impedance of the line was selected as 100 ohms. Theinductance of each section of a ten-section line was 25 millihenrys, andthe capacity of each section 2.5 microfarads. With a source of supplyof, for example, 1000 volts for charging the line with an internalresistance of 900 ohms, and with conventional charging from one end ofthe line, the voltage on the line would rise to about 200 volts during atime interval equal to the discharge period. However, by using the diodesystem of charging, the voltage would be of the order of 360 volts andthere would be absent transients which arise with conventional chargingmethods. Further information about design of transmission lines andoperation thereof will be found in standard texts, such as Principles ofRadar by the M. I. T, Radar School Staff (second edition 1946,McGraW-Hill) chapters 2 and 6.

Referring now to Fig. 4, there is disclosed a system for charging, thedelay line 50 from an alternating current source. More particularly, afull wave thyratron rectifier I00 comprising thyratrons IIlI and I02 areenergized from an A. C. source I03 connected to primary terminals of atransformer I04. The anodes of thyratrons IN and I82 are connected tothe secondary terminals I05 and IE6 respectively of the transformer I04.Conductor I0! is connected between the center top of transformer I04 andthe negative terminal of delay line 50. The cathodes of thyratrons I0!and I02 are connected together and to the positive terminal of delayline 50 by way of conductor 08. A resistor I09 and bias battery I I0 areconnected in series between grid and cathode of thyratron I'M. A similarcircuit is provided for tube E02. A switch I II under control of cam II2 serves effectively to remove battery Hi] from the grid circuit oftube IOI rendering it conductive over the positive half of the potentialcycle of source I00. Similarly, cam II3. driven in synchronism with camH2 from motor 21a actuates switch II4 to control the conduction periodof tube I02. In order to eliminate the possibility of the chargingcircuit being energized during any portion of the discharge period, theswitches III and H4 must be opened by action of cams H2 and H3 prior tothe initiation of discharge from the delay line 50 at a time at leastequal to, and preferably greater than, the period of one-half cycle ofthe voltage from source I05. The'manner in which this is accomplishedwill be explained after describing an electronic reversing switchused inthis modification of the invention.

In the'modification of Fig. 4, the discharge path for the delay line hasbeen illustrated as including an electronic or gas tube reversing switchor circuit controller I I-a;

The gas tube circuit reversing means I la includes four thyratrons I20,IZI, I22 and I23. The plates of the thyratrons I20 and I22 areinterconnected by conductor I25. One terminal of the output side of thedelay line 50 is connected to the plates I20 and I 22 by conductor I 28.In a similar manner, the cathodes of tubes I2I and are connectedtogether and to the other output terminal of the delay line 50 byconductor The cathode of tube I and the plate of tube are interconnectedand coupled by way of conductor 29 to the electrode I2. Likewise theplate of tube IZI and cathode of tube I22 are interconnected and incircuit with the electrode It by way of conductor I30. The grid-cathodecircuit of each of the tubes I20I23 includes a resistance in series witha source of bias voltage, such as the resistor I3I and battery I32included in the grid cathode circuit of tube I20. There is furtherprovided a shorting or bias disabling circuit including switch I33, theterminals of which are connected between grid and cathode. Further, foreach grid circuit there is a switchactuating mechanism driven through aspeed reduction mechanism 271) by the motor 21a. The speed reductionmechanism 211) causes the shaft represented by dotted line I44 to rotateat oneflalf the speed of motor 21a. The actuating mechanisms of Fig. lhave been illustrated as cam surfaces I40, I4I, I42 and I43 coupled byway of driving connection I44 to the motor 27a. Upon rotation of thecams or cam surfaces Mil-- MS, grid-shortin switches, such as the switchI in each of the grid circuits of the gas tube switch, are closed forconduction through the tube associated with each cam. The cams I 143preferably are phased so that switches actuated by cams I40 and MI areactuated, that is, closed or opened 180 from those closed or opened bycams I42 and I43. Further, the cams are spaced from the contact carryingarms of their associated switches so that the bias batteries I32 1 t arebut momentarily effectively removed from the grid circuit during eachrotation. The action is such that thyratrons I20 and I2I will conductsimultaneously and during the period in which thyratrons I22 and I23 arenon-conductive. The heavy line circuit illustrates paths of current flowupon energization of thyratrons I20 and I2I. A current pulse will flowfrom the transmission line through conductor I26, thyratron I20,conductor I29 to electrode I2. The current then ilows through the earthto the electrode I3, through conductor I30, thyratron I2I, and conductor21 to the other terminal of the delay line During the second half of thecycle and for reversed direction of current flow between electrodes I2and I3, the thyratrons I22 and I23 are conductive.

It is to be understood that the cams H2 and i Iii driven by the motor21a will also be phased for closure of switches III and H4 following thetermination of each of the pulses flowing through the earth E toenergize the line-charging circuit during the interval between pulses.In such case, shaft II5 would rotate at a speed twice that of shaft I44.

It is evident that modifications and substitutions may be made in thesystems above described. Forexample, the reversing switch orcircuit-controller I4 may be replaced by the gas tube circuit-controllerI4a. Further, the members which control the production of the spacedpulses have been illustrated in each of the modifications asmechanically driven cam surfaces. This has been done for convenience andshould not be taken as a limitation since it is well understood by thoseskilled in the art that timed electrical pulses from other sources maybe utilized to initiate conduction in the thyratron tubes included inthe present invention.

It is to be understood that in electrical prospecting methods, it is thepractice to make measurements at several frequencies for each location.For example, such measurements may be made at 2, 4, 8 and 16 cycles persecond. In order for the system of Figs. 3 and 4 so to operate somemeans must be provided for varying the length of the delay line 50 inorder that pulses of diiferent lengths may be produced. This is readilyaccomplished, Fig. 3, by removing or adding, as by a multi-terminalswitch, sections of the line 50. For example, the line may be made up insectional units for ease in switching operations.

Though the invention has been illustrated by several modifications, itis to be understood that further modifications within the scope of theappended claims may now suggest themselves to those skilled in the art.

What is claimed is: v

l. A system for generating spaced low-frequency current pulses ofalternating polarity which comprises a source of spaced unidirectionalcurrent pulses, a pair of current electrodes. means intermediate saidsource and said electrodes for reversing the connections therebetween,and means for actuating said reversing means in the interval betweenspaced unidirectional pulses whereby pulses of one polarity and then ofa reverse polarity flow through said electrodes.

2. A system for generating spaced low-frequency rectangular currentpulses alternately of opposite polarity which comprises a source ofspaced unidirectional rectangular current pulses, a pair of currentelectrodes, means intermediate said source and said electrodes forreversing the connections therebetween, and means for actuating saidreversing means in the interval between said spaced unidirectionalpulses whereby said pulses of alternately reverse polarity flow throughsaid electrodes.

3. A system for generating spaced low-fre quency pulses alternately ofopposite polarity for current flow between a pair of spaced electrodeswhich comprises a direct current circuit, a reversing circuit-controllerintermediate said direct current circuit and said electrodes and acommutator associated with both said direct current circuit and saidreversing controller for energizing said direct current circuit duringpredetermined intervals and to actuate said reversing controller in theinterval between periods of energization of said direct current circuitto produce rectangular current pulses of alternating polarity betweensaid electrodes.

4. In a system for generating low-frequency pulses alternately ofopposite polarity for current fiow between a pair of current electrodes,the combination which. comprises direct current switching means, athyratron circuit connected between said switching means and saidelectrodes, a motor-driven commutator, means coupling said commutatorand said switching means to produce upon rotation of said commutator aseries of unidirectional pulses spaced one from the other, meanscoupling said commutator and said thyratron circuit to energize saidcircuit in the interval between adjacent current pulses to reverse theconnection between said electrodes and said switching means thereby toproduce current pulses alternately of opposite polarity through saidelectrodes.

5. In a system for generating spaced low-frequency current pulsesalternately of opposite polarity for current flow between a pair ofelectrodes in geophysical prospecting the combination which comprises anelectron discharge path, a grid circuit including a source of potentialfor maintaining said discharge path normally non-conductive,circuit-reversing means intermediate said discharge path and saidelectrodes, actuating means for removing a portion of said source ofpotential from said grid circuit to render said discharge pathconductive at spaced intervals, a coupling between said actuating meansand said reversing means for reversing the connections between saidelectrodes and said discharge path in the interval between repeatedremovals of said source of potential for flow of current pulsesalternately of opposite polarity through said electrodes.

6. A system having low power controlling requirements for deliveringhigh-power, low-=frequency, spaced, rectangular current pulses to a loadimpedance comprising a' grid-controlled source of unidirectional spacedrectangular current pulses, a reversing circuit-controller connectingsaid load impedance to said source, and control means for energizingsaid grid-controlled source to produce said spaced pulses and forsynchronously energizing said circuit-controller during the intervalbetween said pulses for flow of spaced rectangular pulses alternately ofopposite polarity through said load impedance.

7. A system having low power controlling requirements for deliveringlow-frequency, spaced, rectangular current pulses to a load impedancecomprising a vacuum discharge path, a source of potential and a pair ofoutput terminals in series circuit relation, means in the grid circuitof said discharge path to maintain said path normally non-conductive, areversing circuit-controller connecting said load impedance to saidoutput terminals, and a commutator driven at said lowfrequency to removea portion of said potential from said grid circuit to produce spaceduntdirectional current pulses through said path and for energizing saidcontroller during the interval between said spaced current pulses forflow of current pulses alternately of opposite polarity through saidload impedance.

8. In a geophysical prospecting system where low-frequency currentpulses alternately of opposite polarity flow between a pair ofelectrodes which comprises a vacuum discharge path, a grid circuit forsaid vacuum discharge path including a positive source and a negativesource of potential, a reversing circuit-controller intermediate saiddischarge path and electrodes, a commutator, means coupling saidcommutator and said grid circuit of said discharge path for removingsaid negative source of potential from said circuit to render saiddischarge path conductive at spaced intervals, and means coupling saidcommutator and said reversing controller for reversing the connectionbetween said electrodes and said discharge path in the interval duringwhich said source of negative potential is in said control circuit,thereby to produce flow of spaced current pulses alternately of oppositepolarity between said electrodes.

9. A system for generating low-frequency pulses alternately of oppositepolarity for current flow between a pair of spaced electrodes ingeophysical prospecting which comprises a source of potential, a highvacuum switch and a pair of output terminals in series with said source,a control circuit for said high vacuum switch including a second sourceof potential, a reversing circuitcontroller connecting said electrodesto said output? terminals, a device for removing said second source ofpotential from said control circuit durin spaced intervals at said 10wfrequency to produce a series of spaced unidirectional current pulsesflowing to said output terminals, a coupling between said device andsaid controller for reversing the connections between said outputterminals during the interval between each of said unidirectional pulsesto produce pulsed current flow through said electrodes alternately ofopposite polarity.

10. A system for generating low-frequency current pulses alternately ofopposite polarity for flow in a load which comprises a delay-line sourceof spaced unidirectional current pulses, a reversing switch intermediatesaid delay-line source and said load, and means for actuating saidreversing switch in the interval between said spaced unidirectionalpulses to reverse the connections between said load and said delay-linesource for flow of pulses alternately of opposite polarity through saidload.

11. A system for generating low-frequency current pulses alternately ofopposite polarity for flow in a load which comprises a plurality ofseries inductances and shunt condensers forming an artificialtransmission line, a normally open chargin circuit for said lineincluding a source of potential, diodes individually in circuit witheach of said condensers and said source of potential, circuit-reversingmeans intermediate said line and said load, and a circuit-controller forclosing said charging circuit and for energizing said reversing means attwice the frequency of said current pulses alternately to charge saidline from said source and to discharge said line through said load.

12. A system for generating low-frequency current pulses alternately ofopposite polarity for how in a load which comprises a delay lineincluding a plurality of capacitive elements, a circuit including aplurality of diodes for individually charging each capacitive element insaid delayline to a predetermined voltage, a thyratron connected to saiddelay line for controlling discharge thereof, a reversingcircuit-controller connecting said load through said thyratron tosaid'del'ay'line, a control device for alternately energizing saidthyratron and said charging circuit at a rate twice that of saidlow-frequency impulses, and means for operating said controller toreverse said connection between said electrodes and said delay lineduring energization of said charging circuit.

13. square wave-pulse generating circuit comprlsmg' a transmission linehaving a plurality oicharge-storing elements for forming and determmmgthe duration of said pulses, a normally open circuit including asource of potential, an electron discharge path connected between eachof said storing elements of said line and said source of potential, aload impedance and an electron discharge device having a conductiveatmosphere connected in series with each other and with said line, meansassociated with both said normally open circuit and said second-nameddischarge device alternately to close said normally open circuit tocharge said line from said source and to discharge said line throughsaid load impedance, circuit-reversing means interposed between saidline and said load impedance and means for actuating saidcircuit-reversing means during the period of charging of said line 14. ncircuit for generating a series of pulses comprising a transmission lineincluding a plu new, 1 so rality o1 energy-storing elements fordetermining said pulse period, a charging circuit including a source ofpotential, an electron discharge device connected between each of saidelements and said source of potential, a load impedance and a thyratronin series circuit with said line, a circuit-controll r for alternatelyclosing said charging circuit and rendering said thyratron conductive toproduce said series of pulses in said load impedance, circuit-reversingmeans connected between said load impedance and said thyratron, andmeans for actuating said circuit-reversing means during the time ofclosure of said charging circuit.

15.,A circuit for generating a series of pulses comprising a pluralityof shunt condensers and series inductances forming a transmission line,a charging circuit for said line including a source of potential,switching means, and diodes equal in number to condensers with theirplates connected in parallel, a connection between each of saidcondensers and the cathode of one of said diodes, a discharge circuitfor said line including a load impedance and a thyratron,circuitcontrolling means alternately to connect said diodes and saidcondensers to said source to charge said line and to fire said thyratronfor discharge of said line through said load impedance,circuit-reversing means connected between said load impedance and saidthyratron, and means for actuating said circuit-reversing means duringthe charging of said line and prior to the firing of said thyratron.

16. A current-supply system for electrical prospecting which comprises aload, a source of current supply for said load, a reversingcircuitcontroller connected between said current source and said lead,said source including electric valve means for producing a succession ofrectangular current pulses each spaced one from the other by apredetermined time interval, and means for operating said reversingcontroller during each said time interval for application to said loadof rectangular impulses of successively opposite polarity.

17. A current-supply system which comprises a reversingcircuit-controller for reversing the connections to a load, atransmission line having output terminals connected to said reversingcontroller and comprising a plurality of interconnccted capacitors andinductances, a charging circuit individual to each of said capacitors, adischarge circuit separate from said individual charging circuits andincluding said reversing controller, said capacitors and inductancespredetermining the length of a current impulse deiivered to saidcharging circuit, means for repeatedly closing said discharge circuitwith time intervals between each closure thereof, means for 16 operatingsaid reversing controller during each interval of time betweensuccessive closures of said discharge circuit, and means for completingsaid individual charging circuits during said spaced intervals of time.

18. A current-supply system which comprises a source of current supplyfor a load, a reversing switch connected between said current source andsaid load, said source including electric valve means for producing asuccession of rectangular current pulses each spaced one from the otherby a predetermined time interval, and means operating said reversingswitch from one position to the other during each said time interval.for application to said load of rectangular impulses of successivelyopposite polarity.

19. A current-supply system which comprises a reversing switch forreversing the connections to a load, a transmission line having outputterminals connected to said reversing switch and comprising a pluralityof interconnected capacitors and inductances, a charging circuitindividual to each of said capacitors, a discharge circuit separate fromsaid individual charging circuits and including said reversing switchand said load, said capacitors and inductances predetermining the lengthof a current impulse delivered to said load, means for repeatedlyclosing said circuit to said load with time intervals between eachclosure thereof, means for operating said reversing switch for movementfrom one position to the other during each interval of time betweensuccessive closures of said load discharge circuit, and meanssimultaneously completing said individual charging circuits during saidspaced intervals of time for uniformly elevatin the voltage on all ofthe capacitors of said line for producing upon closure of said loadcircuit at any time an electrical pulse of rectangular shape.

GERALD C. SUMMERS.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Re. 21,784 Borden Apr. 29, 1941 2,006,582 Callahan July 2,1935 2,046,436 Wascheck July '7, 1936 2,114,298 Gunn Apr. 19, 19382,303,968 White Dec. 1, 1942 2,342,629 Evjen et al 29, 1944 2,375,778Evjen 1 May 15, 2,438,962 Burlingarne Apr. 6, 1948 2,509,017 Sear May23, 1950 2,511,881 Snyder June 20, 1950 FOREIGN PATENTS Number CountryDate 579,679 Great Britain Aug. 12, 1946

